I think it's both instant and, more importantly, evolving. I think you need instant chemistry to start a relationship, but that alone is not going to hold things together for very long. My husband and I started dating more than 6 years before we married. In that time, we broke up a couple of times... and each decided to give things another chance each time. We are happily married now -- we needed those times for ourselves to mature and for our relationship to grow. So nope, I don't regret it.
(Not gonna go into the answer to this question about my former long-term relationship, because obviously that one was a fail!)
And the couple of times I've fallen in love, I noticed something I liked about them instantly.
I made this journal so that I can join certain communities without anyone I know knowing.